In aerial reconnaissance, in order to cover a large amount of area in a single exposure a wide angle camera is required. Not only must wide angle optics be used, in order to resolve small objects, an extremely large focal plane array is required to capture the full scene shot by the wide angle camera. Thus, wide field of view optical systems or cameras require extremely large focal plane detector arrays.
It is noted that extremely large focal plane detector arrays cannot be fabricated monolithically to capture the entire field of view in one exposure. The result is that one must use a mosaic of limited size focal plane detector arrays.
For instance, a wide angle high resolution camera might have a lens which is 50.8 centimeters (20 inches) in diameter and would require a focal plane array of between 50.8 and 101.6 centimeters (20 and 40 inches) on a side. Moreover, the high resolution involved in capturing motion oftentimes requires at least a half billion pixels. This high resolution pixel density can only be provided through the use of a mosaic of smaller high pixel density focal plane array detectors.
The mosaic however has gaps or dark stripes between the small detector arrays. It is in these gaps that image data is lost because there are no active detectors in the gap. When a point on the image is focused onto a gap or stripe between adjacent detector arrays the image data is lost.
High resolution is required in reconnaissance where one wants to capture all terrain features as well as to be able to detect motion of individuals or vehicles. This scenario requires at least a half billion pixel resolution as well as a wide field of view camera to be able to surveil large areas. For instance, it is desirable to take a single picture with a wide angle camera having a field of view between 40° and 60° and simultaneously capture the full scene on the pixels of a large focal plane array.
Thus, it is important to be able to fabricate a focal plane array with multiple smaller detector arrays, in one case having 5 million detector pixels per array. If one combines 90 of these smaller detector arrays into a mosaic, then the combined array will have as many as 450 million pixels.
This type of resolution is adequate to detect motion of human beings on the ground, but suffers a number of problems, not the least of which are the gaps between the individual detector arrays. The gaps between any two of the adjacent focal plane arrays effectively results in a region or stripe where there are no photo detectors. The result is a checkerboard cut-out of the imaged terrain due to the gaps between the smaller detector arrays. With such a mosaic one can lose one quarter of the image.
Losing one quarter of the information in an image is clearly unacceptable. In the past there have been solutions involving scanning the image across the photo detector array mosaic so that image information is available for the entire full scene, although not simultaneously. Rather, during a scan of the image on the focal plane array, in the past one sequentially samples the array at intervals and uses available image data to fill in the lost data in a concatenation process. The scanning involves moving the image periodically back and forth across the mosaic in a reciprocating scanning fashion.
For full motion capture it is important to have a frame rate of between 5 and 30 frames per second. This means that, without a fully monolithic focal plane array, the scanning must toggle the image on the focal plane array back and forth at least 10 to 60 times per second. The image is scanned or toggled back and forth across the focal plane detector mosaic, such that the image moves for instance by 1° or 2°. This assures that information in the image is not lost because while at one instant of time the focused image may fall on a dark or non-light detecting portion of the array, at another time this same image will have been scanned onto an adjacent active detector chip.
Thus, in order to make a planar photo detector mosaic one must use a number of smaller detector chips which leave dark spaces between the chips where light is not detected. By scanning the image back and forth or up and down across the array mosaic, while simultaneous image detection is not possible, sequential image detection captures all of the available pixels.
In the past, in order to avoid the problem of having focused energy impinge on the dark regions between the small detector arrays, one can utilize two separate cameras having optical axes offset or skewed one with respect to the other. Thus, with two detector arrays having stripes of detectors and two cameras one can simultaneously detect full scene pixel information.
It is thus possible to utilize identical cameras with identical focal plane arrays, with the cameras tilted slightly differently in two dimensions. If for instance the checkerboard arrays were located behind these cameras, then for one direction one would need two separate cameras skewed one with respect to the other to cause an offset in their centerlines of for instance 1°. For orthogonally running gaps or stripes one would need another pair of offset cameras. Thus, four separately tilted cameras would be required. In summary, while simultaneous collection would be possible in this scenario one would need either 2 or 4 times the amount of equipment.
Thus while multiple cameras can provide simultaneous image capture, such a system suffers a cost and weight penalty.
A second way to solve the problem of the dark areas on the checkerboard array is to scan the image across the planar focal plane array by moving the entire camera. The camera must be moved 1° at the very least and sometimes as much as 20° depending on the array utilized. However, the camera and its assembly is a massive assembly requiring the entire payload to be moved in order to provide for the scanning.
While the above technique is effective in obtaining all of the pixilated information from the wide angle camera by taking the data at different times from slightly different angles, there are problems not only due to the mass of the camera but also due to the power necessary to move the camera and the large motors involved. Moreover, when the camera is moved there is a finite settling time such that it takes longer before one can take the next picture.
If one needs to provide frame rates of 5 to 30 frames per second, one can see that moving the camera this fast with the attendant settling time is challenging. Depending on how many snapshots are required to make one frame and assuming a frame rate of for instance 10 frames per second, if one needs 4 exposures to make each picture to eliminate the problem of the array gaps, one would need 40 snapshots per second. Moving a massive assembly to accommodate 40 snapshots per second adds mechanical complexity and weight and power penalties.
Another way to scan the image on a planar focal plane array mosaic is to utilize a scanning mirror. However, while scanning mirrors provide acceptable performance there is an issue having to do with the difficulty of packaging such a scanning mirror assembly given mirror reflections. Further, if one has a large aperture camera one would need a mirror larger than the aperture to capture the full range of fields of view.
For instance, if one has a 12″ diameter lens one might require a large 16″ to 20″ scanning mirror. Add to this the requirement to fold the optical path and size becomes a major issue.
Therefore in order to accommodate mosaic focal planes with a single lens/camera assembly, what one essentially has to do is scan the image across the focal plane array mosaic and take sequential readings so that data which may be lost due to a image being focused on a dark area is now recaptured moments later because it is focused on an active area.
In summary, in typical scanning apparatus, an optical element such as a scan mirror or a Risley prism assembly is placed in front of an optical sensor to re-point the sensor to a new field of regard. However, as mentioned above, such systems add size and weight to the optical system and if one is not simultaneously using multiple cameras or if one is not utilizing a movable camera to scan the wide field of view between two or more discrete fields of regard, then one must come up with a compact economic arrangement, which does not vibrate the camera package or destroy the sharpness of the camera image.